1. Field of the Invention
The present invention relates to the field of data reproduction, and more particularly, to a data reproduction apparatus and method with optimum performance by adjusting filter coefficients for an equalizer to minimize a level error in detecting data.
2. Description of the Related Art
As a technique for increasing recording density by signal processing without changing the characteristics of a conventional recording/reproducing apparatus, partial response maximum likelihood (PRML) related technique, including Viterbi decoding, has advanced.
Viterbi decoders have a predetermined reference level for use in detecting an error value of data. To match the level of input data of a Viterbi decoder to the predetermined reference level, filter tap coefficients of an equalizer connected in front of the Viterbi detector are adjusted to output data whose level matches the reference level of the Viterbi detector.
FIG. 1 illustrates a coefficient adaptation method of a conventional equalizer. An analog-to-digital converter (ADC) 10 samples an input radio frequency (RF) signal, and an asymmetry compensator 12 and an adder 14 correct for asymmetry or a DC offset component of the sample RF signal and provides the result to an equalizer 16. A 3T level error detector 18 detects a difference, i.e., error (ek), between a reference value and the minimum pit (or mark) level, which corresponds to 3T (where T is a pit interval) in a conventional digital versatile disc (DVD) or compact disc (CD)), output from the equalizer 16 which is constituted of a finite impulse response (FIR) filter.
If the error value detected by the 3T level error detector 18 is a positive value, a filter coefficient adjuster 20 determines that the minimum pit level is larger than the reference value. In this case, the filter coefficient adjuster 20 provides the equalizer 16 with a filter coefficient Wk+1, which is adjusted to decrement a filter coefficient Kb and increment a filter coefficient Ka, to decrease the minimum pit level output from the equalizer 16. To the contrary, if the error value detected by the 3T level error detector 18 has a negative value, the filter coefficient adjuster 20 determines that the minimum pit level is smaller than the reference value. As a result, the filter coefficient adjuster 20 provides the equalizer 16 with the filter coefficient Wk+1, which is adjusted to decrement the filter coefficient Ka and increment the filter coefficient Kb, to increase the minimum pit level output from the equalizer 16. The minimum pit level is adjusted to an appropriate level in the above manner, so that the performance of the Viterbi detector 22 is enhanced.
FIG. 2 illustrates the structure of the equalizer 16. In FIGS. 1 and 2, xk indicates data input to the equalizer 16, yk indicates data output from the equalizer 16, and Wk+1 indicates the filter coefficient for the equalizer 16 after coefficient adaptation. Reference numerals 30, 32, 34, 36, 38, 40, 42 and 44 denote delays, reference numerals 46, 48 and 50 denote coefficient multipliers, and reference numeral 52 denotes an adder.
With the structure of the conventional equalizer, detection of the minimum pit level is difficult and level error detection is restricted because only a middle level of the minimum pit is used. Thus, it is difficult to expect a great improvement in reproduction performance. In addition, even though defocusing occurs, the coefficient adaptation in the conventional equalizer ensures the improvement of reproduction performance, by adjusting filter coefficients. However, the conventional equalizer cannot be adopted when the coefficients of the equalizer asymmetrically vary due to tangential tilting.